Method and apparatus for handoff of multiple bearers with differing quality of service levels in a wireless communication system

ABSTRACT

A terminal device and a method of operating a terminal device in a wireless communication network. The method comprises establishing, by the terminal device, a first radio connection with a first network access node for transmission of data via a first bearer and a second radio connection with the first network access node for transmission of data via a second bearer. Based on a difference between quality of service for the data transmission via the first bearer and the second bearer, a third radio connection is established between the terminal device and a second network access node for transmission of the data via the first bearer; and the third radio connection is established between the terminal device and the second network access node, a fourth radio connection is established between the terminal device and the second network access node for transmission of the data via the second bearer.

BACKGROUND Field

The present disclosure relates to wireless communications apparatus andmethods wherein radio connections for supporting the transmission ofdata via two bearers, each having a respective quality of service, areestablished between a terminal device and a network access node.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentinvention.

Third and fourth generation mobile telecommunication systems, such asthose based on the 3GPP defined UMTS and Long Term Evolution (LTE)architecture, are able to support more sophisticated services thansimple voice and messaging services offered by previous generations ofmobile telecommunication systems. For example, with the improved radiointerface and enhanced data rates provided by LTE systems, a user isable to enjoy high data rate applications such as mobile video streamingand mobile video conferencing that would previously only have beenavailable via a fixed line data connection. The demand to deploy suchnetworks is therefore strong and the coverage area of these networks,i.e. geographic locations where access to the networks is possible, maybe expected to increase ever more rapidly.

Future wireless communications networks will be expected to routinelyand efficiently support communications with a wider range of devicesassociated with a wider range of data traffic profiles and types thancurrent systems are optimised to support. For example it is expectedthat future wireless communications networks will be expected toefficiently support communications with devices including reducedcomplexity devices, machine type communication devices, high resolutionvideo displays, virtual reality headsets and so on. Some of thesedifferent types of devices may be deployed in very large numbers, forexample low complexity devices for supporting the “The Internet ofThings”, and may typically be associated with the transmissions ofrelatively small amounts of data with relatively high latency tolerance.Other types of devices may generate or receive categories of dataassociated with differing quality of service requirements—some lowbitrate data may be associated with, for example, a low latency andnear-zero packet loss requirement; other data, having a higher bitrate,may be more tolerant of latency and/or packet loss.

In view of this there is expected to be a desire for future wirelesscommunications networks, for example those which may be referred to as5G or new radio (NR) system/new radio access technology (RAT) systems,as well as future iterations/releases of existing systems, toefficiently support connectivity for a wide range of devices associatedwith different applications and different characteristic data trafficprofiles and data transfer requirements, for example in terms of latencyand/or reliability targets. See, for example, the 3GPP documentRP-160671, “New SID Proposal: Study on New Radio Access Technology,” NTTDOCOMO, RAN#71 [1].

The introduction of new radio access technology (RAT) systems/networksgives rise to new challenges for providing efficient operation fordevices operating in new RAT networks, including devices able to operatein both new RAT networks (e.g. a 3GPP 5G network) and currently deployedRAT networks (e.g. a 3GPP 4G network).

One particular area where new approaches may be helpful is in relationto handovers between network nodes responsible for communicating with aterminal device, which may be referred to as mobility management. Itwill be appreciated that handovers may result from a device physicallymoving between coverage areas of different cells or from changing radioconditions associated with different cells for a static device, and theterm mobility management may be used for both scenarios.

In view of the above, there is a desire for new approaches for handlingmobility in wireless communications network.

SUMMARY

The present disclosure can help address or mitigate at least some of theissues discussed above.

Respective aspects and features of the present disclosure are defined inthe appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the present technology. The described embodiments,together with further advantages, will be best understood by referenceto the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein likereference numerals designate identical or corresponding parts throughoutthe several views, and wherein:

FIG. 1 schematically represents some aspects of a LTE-type wirelesstelecommunication network which may be configured to operate inaccordance with certain embodiments of the present disclosure;

FIG. 2 schematically represents some aspects of a new radio accesstechnology (RAT) wireless communications network which may be configuredto operate in accordance with certain embodiments of the presentdisclosure;

FIG. 3 schematically represents components of a head-mounted displaywhich may be configured to operate in accordance with certainembodiments of the present disclosure;

FIG. 4 schematically represents aspects of a wireless communicationsnetwork which may be configured to operate in accordance with certainembodiments of the present disclosure;

FIG. 5 is a signalling diagram representing aspects of a handoverprocedure in accordance with certain embodiments of the presentdisclosure;

FIG. 6 schematically represents aspects of a handover procedure inaccordance with certain embodiments of the present disclosure;

FIG. 7 is a signalling diagram representing aspects of a handoverprocedure in accordance with certain embodiments of the presentdisclosure;

FIG. 8 is a signalling diagram representing aspects of a handoverprocedure in accordance with certain embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 provides a schematic diagram illustrating some basicfunctionality of a wireless communications network/system 100 operatinggenerally in accordance with LTE principles, but which may also supportother radio access technologies, and which may implement embodiments ofthe disclosure as described herein. Various elements of FIG. 1 andcertain aspects of their respective modes of operation are well-knownand defined in the relevant standards administered by the 3GPP (RTM)body, and also described in many books on the subject, for example,Holma H. and Toskala A [2]. It will be appreciated that operationalaspects of the wireless communications networks discussed herein whichare not specifically described (for example in relation to specificcommunication protocols and physical channels for communicating betweendifferent elements) may be implemented in accordance with any knowntechniques, for example according to the relevant standards and knownproposed modifications and additions to the relevant standards.

The network 100 includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from terminal devices104. Data is transmitted from base stations 101 to terminal devices 104within their respective coverage areas 103 via a radio downlink Data istransmitted from terminal devices 104 to the base stations 101 via aradio uplink. The core network 102 routes data to and from the terminaldevices 104 via the respective base stations 101 and provides functionssuch as authentication, mobility management, charging and so on.Terminal devices may also be referred to as mobile stations, userequipment (UE), user terminal, mobile radio, communications device, andso forth. Base stations, which are an example of network infrastructureequipment/network access node, may also be referred to as transceiverstations/nodeBs/e-nodeBs, g-nodeBs and so forth. In this regarddifferent terminology is often associated with different generations ofwireless communications systems for elements providing broadlycomparable functionality. However, certain embodiments of the disclosuremay be equally implemented in different generations of wirelesscommunications systems, and for simplicity particular terminology may beused regardless of the underlying network architecture. That is to say,the use of a specific term in relation to certain exampleimplementations is not intended to indicate these implementations arelimited to a certain generation of network that may be most associatedwith that particular terminology.

FIG. 2 is a schematic diagram illustrating a network architecture for anew RAT wireless communications network/system 300 based on previouslyproposed approaches which may also be adapted to provide functionalityin accordance with embodiments of the disclosure described herein. Thenew RAT network 300 represented in FIG. 2 comprises a firstcommunication cell 301 and a second communication cell 302. Eachcommunication cell 301, 302, comprises a controlling node (centralisedunit) 321, 322 in communication with a core network component 310 over arespective wired or wireless link 351, 352. The respective controllingnodes 321, 322 are also each in communication with a plurality ofdistributed units (radio access nodes/remote transmission and receptionpoints (TRPs)) 311, 312 in their respective cells. Again, thesecommunications may be over respective wired or wireless links Thedistributed units 311, 312 are responsible for providing the radioaccess interface for terminal devices connected to the network. Eachdistributed unit 311, 312 has a coverage area (radio access footprint)341, 342 which together define the coverage of the respectivecommunication cells 301, 302.

In terms of broad top-level functionality, the core network component310 of the new RAT communications network represented in FIG. 2 may bebroadly considered to correspond with the core network 102 representedin FIG. 1, and the respective controlling nodes 321, 322 and theirassociated distributed units/TRPs 311, 312 may be broadly considered toprovide functionality corresponding to the base stations 101 of FIG. 1.The term network infrastructure equipment/access node may be used toencompass these elements and more conventional base station typeelements of wireless communications systems. Depending on theapplication at hand the responsibility for scheduling transmissionswhich are scheduled on the radio interface between the respectivedistributed units and the terminal devices may lie with the controllingnode/centralised unit and/or the distributed units/TRPs.

A terminal device 400 is represented in FIG. 2 within the coverage areaof the first communication cell 301. This terminal device 400 may thusexchange signalling with the first controlling node 321 in the firstcommunication cell via one of the distributed units 311 associated withthe first communication cell 301. In some cases communications for agiven terminal device are routed through only one of the distributedunits, but it will be appreciated in some other implementationscommunications associated with a given terminal device may be routedthrough more than one distributed unit, for example in a soft handoverscenario and other scenarios. The particular distributed unit(s) throughwhich a terminal device is currently connected through to the associatedcontrolling node may be referred to as active distributed units for theterminal device. Thus the active subset of distributed units for aterminal device may comprise one or more than one distributed unit(TRP). The controlling node 321 is responsible for determining which ofthe distributed units 311 spanning the first communication cell 301 isresponsible for radio communications with the terminal device 400 at anygiven time (i.e. which of the distributed units are currently activedistributed units for the terminal device). Typically this will be basedon measurements of radio channel conditions between the terminal device400 and respective ones of the distributed units 311. In this regard, itwill be appreciated that the subset of the distributed units in a cellwhich are currently active for a terminal device will depend, at leastin part, on the location of the terminal device within the cell (sincethis contributes significantly to the radio channel conditions thatexist between the terminal device and respective ones of the distributedunits).

In the example of FIG. 2, two communication cells 301, 302 and oneterminal device 400 are shown for simplicity, but it will of course beappreciated that in practice the system may comprise a larger number ofcommunication cells (each supported by a respective controlling node andplurality of distributed units) serving a larger number of terminaldevices.

It will further be appreciated that FIG. 2 represents merely one exampleof a proposed architecture for a new RAT communications system in whichapproaches in accordance with the principles described herein may beadopted, and the functionality disclosed herein may also be applied inrespect of wireless communications systems having differentarchitectures.

Thus certain embodiments of the disclosure as discussed herein may beimplemented in wireless telecommunication systems/networks according tovarious different architectures, such as the example architectures shownin FIGS. 1 and 2. It will thus be appreciated the specific wirelesscommunications architecture in any given implementation is not ofprimary significance to the principles described herein. In this regard,certain embodiments of the disclosure may be described generally in thecontext of communications between network infrastructureequipment/access nodes and a terminal device, wherein the specificnature of the network infrastructure equipment/access node and theterminal device will depend on the network infrastructure for theimplementation at hand. For example, in some scenarios the networkinfrastructure equipment/access node may comprise a base station, suchas an LTE-type base station 101 as shown in FIG. 1 which is adapted toprovide functionality in accordance with the principles describedherein, and in other examples the network infrastructureequipment/access node may comprise a control unit/controlling node 321,322 and/or a TRP 311, 312 of the kind shown in FIG. 2 which is adaptedto provide functionality in accordance with the principles describedherein.

As already noted, mobile communications networks such as the network 100shown in FIG. 1 and the network 300 shown in FIG. 2 may be expected tosupport a wide range of services having different requirements, forexample in terms of data rate, latency and/or reliability targets forthe different services, possibly associated with a single terminaldevice. One example service currently considered to be of interest fornext generation wireless communication systems includes so-called UltraReliable and Low Latency Communications (URLLC) [1].

URLLC services may be typically characterised as low latency services,for example aiming to transmit relatively small amounts of data throughthe radio network with a 1 ms packet transit time i.e. so that eachpiece of URLLC data needs to be scheduled and transmitted across thephysical layer in a time that is shorter than 1 millisecond, in order topermit and end-to-end latency between a terminal device's applicationlayer and the edge of the wireless communication network operator'spacket network of no more than 1 millisecond). URLLC services typicallymay require high reliability of data transmission, for example with a99.999% reliability target. URLLC services may, for example, beapplicable for safety-relevant communications, for example,communications relating to autonomous vehicle (driverless car)applications.

An example of a source of data which may require low latencytransmission, as may be provided by a URLLC service, is an inertialmeasurement unit (IMU) associated with a head-mounted display (HMD) forproviding a virtual, augmented, or mixed reality experience.

FIG. 3 illustrates an example HMD 600. The HMD 600 is an image or videodisplay device which may be worn on the head or as part of a helmet.Either one eye or both eyes are provided with small electronic displaydevices.

Some HMDs allow a displayed image to be superimposed on a real-worldview. This type of HMD can be referred to as an optical see-through HMDand generally requires the display devices to be positioned somewhereother than directly in front of the user's eyes. Some way of deflectingthe displayed image so that the user may see it is then required. Thismight be through the use of a partially reflective mirror placed infront of the user's eyes so as to allow the user to see through themirror but also to see a reflection of the output of the displaydevices. In another arrangement, disclosed in EP-A-1 731 943 andUS-A-2010/0157433, the contents of which are incorporated herein byreference, a waveguide arrangement employing total internal reflectionis used to convey a displayed image from a display device disposed tothe side of the user's head so that the user may see the displayed imagebut still see a view of the real world through the waveguide. Onceagain, in either of these types of arrangement, a virtual image of thedisplay is created (using known techniques) so that the user sees thevirtual image at an appropriate size and distance to allow relaxedviewing. For example, even though the physical display device may betiny (for example, 10 mm×10 mm) and may be just a few millimetres fromthe user's eye, the virtual image may be arranged so as to be perceivedby the user at a distance of (for example) 20 m from the user, having aperceived size of 5 m×5 m.

Other HMDs, however, allow the user only to see the displayed images,which is to say that they obscure the real world environment surroundingthe user. This type of HMD can position the actual display devices infront of the user's eyes, in association with appropriate lenses whichplace a virtual displayed image at a suitable distance for the user tofocus in a relaxed manner—for example, at a similar virtual distance andperceived size as the optical see-through HMD described above. This typeof device might be used for viewing movies or similar recorded content,or for viewing so-called virtual reality content representing a virtualspace surrounding the user. It is of course however possible to displaya real-world view on this type of HMD, for example by using aforward-facing camera to generate images for display on the displaydevices.

An inertial measurement unit (IMU) such as the IMU 602, which may beincorporated in the HMD 600, may comprise an accelerometer andgyroscope, each operating in 3 degrees of freedom (DOF). An IMU may thusestimate a change in a user's position, and may be capable of doing sowith very low latency.

The HMD 600 may include processor circuitry 610 which may control thecamera 606, IMU 602 and video display 604.

The HMD 600 may include, or be connected to, a terminal device 608 andmay thus be connected to a wireless communications network, such asthose illustrated in FIG. 1 and FIG. 2.

As described above, the HMD 600 may also incorporate a display 604and/or a camera 606. Video data generated by the camera 606 and/or videodata for display on the display 604 may also be communicated via thewireless communications network by means of the terminal device 608.

The terminal device 608 comprises transmitter circuitry 608 a fortransmission of wireless signals, receiver circuitry 608 b for thereception of wireless signals and processor circuitry 608 c (which mayalso be referred to as a processor/processor unit) configured to controlthe terminal device 608. The processor circuitry 608 c may comprisevarious sub-units/sub-circuits for providing functionality in accordancewith embodiments of the present disclosure as described herein. Thesesub-units may be implemented as discrete hardware elements or asappropriately configured functions of the processor circuitry. Thus theprocessor circuitry 608 c may comprise circuitry which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless communications networks. The transmitter circuitry608 a, receiver circuitry 608 b and the processor circuitry 608 c areschematically shown in FIG. 3 as separate elements for ease ofrepresentation. However, it will be appreciated that the functionalityof these circuitry elements can be provided in various different ways,for example using one or more suitably programmed programmablecomputer(s), or one or more suitably configured application-specificintegrated circuit(s)/circuitry/chip(s)/chipset(s). Where the terminaldevice 608 is integrated in the HMD 600 (as shown in FIG. 3), theprocessor circuitry 608 c and 610 may be common (i.e. common processorcircuitry may provide the functionality of processor circuitry 608 c andprocessor circuitry 610). It will be appreciated the terminal device 608and HMD 600 will in general comprise various other elements associatedwith its operating functionality, for example a power source, userinterface, and so forth, but these are not shown in FIG. 3 in theinterests of simplicity.

The transmission of data generated by the IMU may require a very lowlatency, for example, of around 1 millisecond or lower when measuredbetween the HMD and the edge of the wireless communications network, andwith substantially no packet loss.

As described above, the HMD 600 may also incorporate a display 604and/or a camera 606. Video data generated by the camera 606 and/or videodata for display on the display 604 may also be communicated via thewireless communications network by means of a terminal device 608 whichmay be connected to, or integrated into the HMD 600.

Video data for transmission to or from the HMD 600 may be associatedwith quality of service requirements which provide for the transmissionof several Mbps up to around 1 Gbps with an end-to-end latency of around15 milliseconds or less. The loss of video data is less problematic thanthe loss of IMU data, but there may nevertheless be a packet lossconstraint on the video data.

An HMD may generate or receive additional data, that data beingassociated with yet further quality of service requirements.

The techniques described herein may provide data services meeting thevarious quality of service requirements described above in respect of anHMD during a handover, however it will be readily appreciated that thetechniques described herein are not so limited, but may be applied inany appropriate scenario, including other augmented, virtual, or mixedreality applications, or indeed any other services not associated withvideo display.

Embodiments of the present technique can provide a terminal device and amethod of operating a terminal device in a wireless communicationnetwork. The wireless communication network comprises a first networkaccess node associated with a first cell of the wireless communicationnetwork and a second network access node associated with a second cellof the wireless communication network. The method comprisesestablishing, by the terminal device, a first radio connection betweenthe terminal device and the first network access node for supporting thetransmission of data via a first bearer and a second radio connectionbetween the terminal device and the first network access node forsupporting the transmission of data via a second bearer. In accordancewith a difference between a quality of service for the data transmissionvia the first bearer and a quality of service for the data transmissionvia the second bearer, a third radio connection is established betweenthe terminal device and the second network access node for supportingthe transmission of the data via the first bearer; and the third radioconnection is established between the terminal device and the secondnetwork access node, a fourth radio connection is established betweenthe terminal device and the second network access node for supportingthe transmission of the data via the second bearer. As will beappreciated different bearers may require different qualities of serviceeven for the same application. For some bearers the quality of servicerequirements are higher than others such as, for example, URLLC whichmay be associated with a very low latency requirement and a very lowtolerance of packet loss. For such bearers, embodiments of the presentinvention provide for a handover establishing a radio connection morequickly via a target access node and which can, in some examples,provide dual connectivity so that more reliable communication and/orlower latency communication can be effected during the handover. Havingtwo radio connections active contemporaneously requires greaterresources on the radio interface, and additional processing in both theterminal device and in the wireless communication network. For otherbearers of the terminal device having less stringent quality of servicerequirements, embodiments of the present technique may perform ahandover which differs in some manner from the handover mechanism usedin respect of the first bearer, for example, in the timing, the use (ornot) of dual connectivity or the criteria for establishing radioconnections with the target access node.

The inventors have recognised that applications, such as HMDapplications, may be associated with various sources of data, with thedata being associated with (that is, having) differing quality ofservice requirements. In this regard, it will be recognised that when itis necessary to change the serving cell of a device, such as the HMD600, during ongoing data transmission it is necessary to ensure that thevarious quality of service requirements can be satisfied during thehandover process. In this regard it will be appreciated that handoversmay result from a device physically moving between coverage areasassociated with different radio access nodes or from changing radioconditions associated with different cells for a static device.

In a handover procedure a terminal device is moved (handed over) fromcommunicating with a first network access node (e.g. an LTE base stationor 5G controlling unit) to communicating with a second network accessnode (e.g. another LTE base station or 5G controlling unit). The firstnetwork access node may be referred to as the source for the handoverand the second network access node may be referred to as the target forthe handover. A terminal device will typically be handed over from asource network access node to a target network access node becausechanging radio channel conditions mean the target network access node isbetter able to serve the terminal device than the source network accessnode, e.g. because the terminal device is moving. However, a handovermay also be initiated for other reasons, for example for load balancing.

Many wireless communications systems perform a handover in respect ofall ongoing data communications of a given terminal device as part of asingle process. The inventors have recognised that, when a singleterminal device is supporting one or more applications which generate orreceive data traffic having different quality of service requirements,this approach can be detrimental.

FIG. 4 schematically represents some aspects of a wirelesscommunications system 200 configured to operate to support a terminaldevice 608 approaching a handover from a source network access node 204to a target network access node 206 in accordance with certainembodiments of the disclosure. Aspects of the architecture and operationof the communications system 200 which are not specifically describedherein may be implemented in accordance with any previously proposedtechniques, for example according to current 3GPP standards and otherproposals for operating wireless communications systems/networks. Thenetwork access nodes 204, 206 may, for convenience, sometimes bereferred to herein as base stations 204, 206, it being understood thisterm is used for simplicity and is not intended to imply the networkaccess nodes should conform to any specific network architecture, but onthe contrary, these elements may correspond with any networkinfrastructure equipment/network access node that may be configured toprovide functionality as described herein. In that sense it willappreciated the specific network architecture in which embodiments ofthe disclosure may be implemented is not of primary significance to theprinciples described herein.

The communications system 200 comprises a core network part (evolvedpacket core) 202 coupled to a radio network part. The radio network partcomprises the source network access node 204, the target network accessnode 206, and the terminal device 608. In this example, two networkaccess nodes 204, 206 and one terminal device 608 are shown forsimplicity, but it will of course be appreciated that in practice theradio network part may comprise a larger number of base stations servinga larger number of terminal devices across various communication cells.

As with a conventional mobile radio network, the terminal device 608 isarranged to communicate data to and from the network access nodes (basestations/transceiver stations) 204, 206. Typically the terminal devicewill be connected to (i.e. able to exchange user plane data with) onenetwork access node at a time. However, a terminal device may in somecases be simultaneously connected to both the first and second networknodes, in particular when it is operating in a region where there is apotential for a handover to occur, i.e. when the terminal device is in aboundary region between the geographic footprints associated with theradio coverage (cells) of the respective network nodes. The networkaccess nodes 204, 206 are communicatively connected via respectivecommunication links 217, 218 to the core network part 202, and inparticular to a serving gateway, S-GW, 230 in the core network part 202arranged to perform routing and management of mobile communicationsservices to the terminal devices in the communications system 200 viathe network access nodes 204, 206. In order to maintain mobilitymanagement and connectivity, the core network part 202 also includes amobility management entity, MME, 220 which manages the enhanced packetservice, EPS, connections with the terminal device 608 operating in thecommunications system based on subscriber information stored in a homesubscriber server, HSS. Other network components in the core network inthis example implementation (not shown for simplicity) include a policycharging and resource function, PCRF, and a packet data network gateway,PDN-GW, which provides a connection from the core network part 202 to anexternal packet data network, for example the Internet. As noted above,the operation of the various elements of the communications system 200shown in FIG. 4 may be broadly conventional apart from where modified toprovide functionality in accordance with embodiments of the presentdisclosure as discussed herein.

The source network infrastructure element/access node 204 comprisestransceiver circuitry 204 a (which may also be referred to as atransceiver/transceiver unit) for transmission and reception of wirelesssignals and processor circuitry 204 b (which may also be referred to asa processor/processor unit) configured to control the source networkaccess node 204 to operate in accordance with embodiments of the presentdisclosure as described herein. The processor circuitry 204 b maycomprise various sub-units for providing functionality in accordancewith embodiments of the present disclosure as explained further herein.These sub-units may be implemented as discrete hardware elements or asappropriately configured functions of the processor circuitry. Thus, theprocessor circuitry 204 b may comprise circuitry which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless communications systems. The transceiver circuitry204 a and the processor circuitry 204 b are schematically shown in FIG.4 as separate elements for ease of representation. However, it will beappreciated that the functionality of these circuitry elements can beprovided in various different ways, for example using one or moresuitably programmed programmable computer(s), or one or more suitablyconfigured application-specific integratedcircuit(s)/circuitry/chip(s)/chipset(s). It will be appreciated thesource network access node 204 will in general comprise various otherelements associated with its operating functionality, such as ascheduler. For example, although not shown in FIG. 4 for simplicity, theprocessor circuitry 204 b may comprise scheduling circuitry, that is tosay the processor circuitry 204 b may be configured/programmed toprovide the scheduling function for the source network access node 204.

The target network infrastructure element/access node 206 comprisestransceiver circuitry 206 a (which may also be referred to as atransceiver/transceiver unit) for transmission and reception of wirelesssignals and processor circuitry 206 b (which may also be referred to asa processor/processor unit) configured to control the target networkaccess node 206 to operate in accordance with embodiments of the presentdisclosure as described herein. The processor circuitry 206 b maycomprise various sub-units for providing functionality in accordancewith embodiments of the present disclosure as explained further herein.These sub-units may be implemented as discrete hardware elements or asappropriately configured functions of the processor circuitry. Thus, theprocessor circuitry 206 b may comprise circuitry which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless communications systems. The transceiver circuitry206 a and the processor circuitry 206 b are schematically shown in FIG.4 as separate elements for ease of representation. However, it will beappreciated that the functionality of these circuitry elements can beprovided in various different ways, for example using one or moresuitably programmed programmable computer(s), or one or more suitablyconfigured application-specific integratedcircuit(s)/circuitry/chip(s)/chipset(s). For example, although not shownin FIG. 4 for simplicity, the processor circuitry 206 b may comprisescheduling circuitry, that is to say the processor circuitry 206 b maybe configured/programmed to provide the scheduling function for thetarget network access node 206.

Thus for the example implementation scenario represented in FIG. 4 it isassumed the terminal device 608 is connected to the source networkaccess node 204 over a radio path 212 and has moved to a location inwhich it may be handed over to the target network access node 206, e.g.due to mobility, so that it may communicate with the target networkaccess node over the radio path 214. The specific procedure fordetermining if the terminal device should in fact be handed over may bebased on conventional techniques, for example taking account of radiochannel conditions associated with the respective radio paths 212, 214and/or load balancing. A communications link 210 between the networkaccess nodes 204, 206 is provided so they may exchange information, forexample to support a handover procedure. In some network architecturesthe network nodes may communicate directly with one another, asschematically represented in FIG. 4, while in other networkarchitectures they may communicate with one another indirectly via thecore network part 202.

FIG. 5 illustrates a sequence chart showing the principles of thetechniques describes herein. The sequence starts at step s10 in whichthe terminal device 608 has established radio connections with theaccess node A 204 and is in a radio resource configuration (RRC)connected mode. This RRC connected mode provides the radio connectionsfor the transmission of data via bearer 1 and bearer 2.

Not shown in FIG. 5 is an ongoing evaluation of the radio channelsbetween the access node A 204 and the terminal device 608 and also ofradio channels between the terminal device 608 and other access nodessuch as the access node B 206. Based on these ongoing evaluations adecision is made either by the terminal device 608 or by the access nodeA 204 to initiate the mobility procedure for bearer 1 illustrated atstep s20. The mobility procedure for bearer 1 s20 results in theestablishment of a radio connection between the terminal device 608 andthe access node B 206 for supporting the transmission of data via bearer1.

Examples of procedures suitable for carrying out the mobility procedurefor bearer 1 at s20 are provided below.

After the radio connection between the terminal device 608 and theaccess node B 206 for providing for the communication of data associatedwith bearer 1 has been established, a decision is made either by theterminal device 608 or by the access node A 204 to perform the mobilityprocedure for bearer 2 at step s30.

The details of this mobility procedure may be substantially the same asthat illustrated at s20 or may be different. For example, the mobilityprocedure at step s20 may result in a dual connectivity configurationbeing applied to two radio connections supporting bearer 1, while themobility procedure for bearer 2 at s30 may be a conventionalbreak-before-make handover, in which the radio connection between theterminal device 608 and the access node A 204 providing for thetransmission of data associated with bearer 2 is terminatedsubstantially simultaneously with the establishment of a radioconnection between the terminal device 608 and the access node B 206.

As will be readily appreciated, if either or both of the first andsecond mobility procedures (s20, s30) have resulted in a dualconnectivity configuration in respect of either the radio connectionsfor bearer 1 or the radio connections for bearer 2, then subsequentlythe dual connectivity configuration may be terminated. In other words,any radio connection remaining active between the terminal device 608and the access node A 204 may be released and any bearers in the sourcecell may be terminated.

If the first mobility procedure s20 for bearer 1 results in a dualconnectivity configuration, then in some embodiments of the presenttechnique this may be reconfigured to a conventional single radiochannel either substantially at the same time as the initialisation ofthe mobility procedure s30, or prior to that time, or after thecompletion of the mobility procedure s30.

Following the completion of both the mobility procedure s20 and themobility procedure s30 (and the reconfiguration of any dual connectivityconfiguration), the terminal device 608 is in an RRC connected modehaving radio connections with the access node B 206 supporting thetransmission of data corresponding to bearers 1 and 2.

FIG. 5 illustrates the use of the present technique in scenarios wheretwo bearers are established by the terminal device 608. However thepresent technique is not so limited, but may be applied to any number ofbearers greater than one. Accordingly, each mobility procedure may applyto one or more bearers. In some embodiments, there may be more than twomobility procedures. Each mobility procedure (characterised by themobility mechanism, and the triggering criteria) may be selectedaccording to the quality of service requirements associated with thecorresponding bearer(s). In some embodiments, one bearer may beassociated with data generated by an IMU of an HMD; the second bearermay be used for the transmission of video data for the HMD.

FIG. 6 illustrates an example procedure in accordance with the techniquedisclosed herein.

In the procedure illustrated in FIG. 6, the terminal device 608 ismoving along a track indicated by 504, passing through positions P1, P2,P3 and P4, and hence moving from the coverage of a first cell 520associated with a source access node, which may be the network accessnode 204 into the coverage of a second cell 522 associated with a targetaccess node, which may be the network access node 206.

In an initial step, the terminal device 608 is at position P1 and is inRRC Connected mode, having established radio connections 502 with theserving network access node 204 for the support of three bearers,bearers 1 to 3, which support the transmission of data from the terminaldevice 608 to the core network 202.

In a second step, the terminal device 608, moving along a path 504 hasmoved to position P2. Based on an evaluation of the radio conditions forthe terminal device 608 and taking into account the quality of servicerequirements for bearer 1, the serving access node 204 triggers eitheran early handover or a request to establish dual connectivity for bearerone only. In some embodiments of the present technique, resources forradio connections for the support of transmissions via bearers 2 and 3are also reserved in the target access node 206 in the second step.

The serving access node 204 initiates a handover procedure towards thetarget access node 206, and the target access node 206 sets up resourcesfor bearer 1 only. The type of handover procedure which is applied onlyto bearer 1, may depend on the capabilities of the terminal device 608and may be in accordance with one of the techniques described inco-pending application EP 17163832.3, the contents of which are herebyincorporated by reference.

The target access node 206 may determine whether an end to end latencyrequirement for bearer 1 cannot be met if user plane data continues tobe routed via the source access node 204 to the core network user planefunction (for example, via connection 514). In some example embodiments,the source access node 204 may determine whether an end to end latencyrequirement for bearer 1 cannot be met if the user plane data were to berouted from the target access node 206 to the core network user planefunction via the source access node 204. This determination may becarried out before the serving access node 204 initiates the handoverprocedure towards the target access node 206 for bearer 1, and mayindicate the outcome of this determination in the handover preparationsignalling.

Determining whether the end to end latency requirement for bearer 1cannot be met if the user plane data were to be routed from the targetaccess node 206 to the core network user plane function via the sourceaccess node 204 may comprise evaluating the backhaul latency conditionsfor the connections between the source access node 204 and the targetaccess node 206 (e.g. by means of connection 210) and between the targetaccess node 206 and the core network 202. This determination may occurprior to the triggering of a mobility procedure (e.g. before the servingaccess node 204 initiates a handover procedure towards the target accessnode 206).

Based on this determination, the target access node 206 may establish anew connection 512 towards the core network 202 is established betweenthe target access node 206 and a user plane function in the core network202.

As a result, data for bearer 1 is transmitted via both radio connections506 between the terminal device 608 and the source access node 204, andby radio connections 508 between the terminal device 608 and the targetaccess node 206. Data for bearer 2 and bearer 3 is transmitted viaconnections 510 between the terminal device 608 and the source accessnode 204.

In a next step, the terminal device 608 has moved to position P3 and thesource access node 204 determines that handover criteria are met inrespect of the radio connections for bearers 2 and 3. These criteria maybe based on radio channel measurements made by the terminal device 608,radio resource management (RRM) policies of the source access node 204and the quality of service requirements for bearers 2 and 3.

Based on this determination, the source access node 204 initiates aconventional handover procedure for bearers 2 and 3. This results in theestablishment of radio connections 516 between the terminal device 608and the target access node 206 for bearers 2 and 3, and the release ofthe radio connections 510 between the terminal device 608 and the sourceaccess node 204 for bearers 2 and 3. During the handover for bearers 2and 3, radio connections for bearer 1 may remain unchanged.

In a further step, the terminal device 608 has moved to position P4. Theradio connection 506 supporting bearer 1 between the terminal device 608and the source access node 204 is released.

In some embodiments, the release of the radio connection 506 supportingbearer 1 between the terminal device 608 and the source access node 204occurs at substantially at the same time as the release of the radioconnections 510 supporting bearers 2 and 3 between the terminal device608 and the source access node 204.

FIG. 7 illustrates an example producer according to the techniquesdescribed herein. In the example of FIG. 7, the terminal device 608 hasestablished three bearers. The first mobility procedure is applied tobearer 1, and the second mobility procedure is applied to bearers 2 and3.

In some embodiments, bearer 1 may be configured to transmit datagenerated by an IMU in an HMD and thus (or otherwise) be associated witha requirement for very low latency transmission. In some embodiments,bearer 2 may be associated with video data either generated by a cameraintegrated with an HMD or for display on a display screen of an HMD, orboth; bearer 2 may thus have quality of service requirements appropriatefor video data (which may be characterized by requirements for one ormore of a high bandwidth, a relatively low packet loss, and a lowdelay). In some embodiments, bearer 3 may be used for other datagenerated by, or for the use of, a processor associated with an HMD.

The procedure starts at s100 in which the terminal device 608 is in anRRC connected mode and has ongoing radio connections with the accessnode A 204 providing for the transmission of data associated withbearers 1, 2 and 3. Data from the access node A 204 is transmitted tothe core network 202 by means of one or more connections between theaccess node A 204 and the core network 202.

At step s102 reference signals are transmitted on an ongoing basis byeach access node. These reference signals are received and measuredperiodically by the terminal device 608. This measurement of radiochannel conditions may, for example, comprise measurements of referencesignal received power, RSRP, and/or reference signal received quality,RSRQ. In general, the process of establishing measurements of radiochannel conditions between the terminal device and the different networkaccess nodes may be performed in accordance with conventionaltechniques. Furthermore, it will be appreciated that while FIG. 7schematically represents reference signals being transmitted only once(in step s102), in practice, and as is well established, referencesignals will be transmitted on an ongoing/continuous basis throughoutthe processing represented in FIG. 7.

Based on the evaluation of the reference signals the terminal device 608may transmit at step s104 a measurement report to the serving accessnode, that is, the access node A 204. The access node A 204 may evaluatethe measurement report received at s104. Alternately or additionally,the access node A 204 may monitor the radio channels between theterminal device 608 and the access node A 204 based on received signalstransmitted by the terminal device 608.

Based on these evaluations, the access node A 204 may initiate ahandover in respect of one of the bearers. In the example shown in FIG.7 the access node A 204 initiates a handover for the radio connectionsupporting the transmission of data associated with bearer 1, bytransmitting a handover preparation message s106 to the selected targetaccess node, that is, the access node B 206.

In response to receiving the handover preparation message s106, theaccess node B 206 may establish, at steps s108 and s110, a connectionwith the core network 202, in order to support the transmission of dataassociated with bearer 1 of the terminal device 608. After thisconnection is established, the access node B 206 responds by means of anacknowledgement to the access node A 204 in step s112.

In some embodiments of the present technique, the access node B 206 maydetermine whether or nor it is necessary to establish a connection withthe core network 202. This determination may be based on the quality ofservice (and particularly, latency) requirements for the bearer(s)subject to the mobility procedure. In particular, if it has beendetermined (either by the access node B 206 or by the access node A 204)that routing user plane data to the core network 202 via the access nodeA 204 may result in the latency requirement not being met, then theaccess node B 206 may initiate the establishment of a direct connectionwith the core network 202 by means of steps s108 and s110.

In some embodiments of the present technique, the access node B 206performs admission control (not shown in FIG. 7) to determine whether itis able to accept the handover request. This ensures that, for example,sufficient radio resources are available at the access node B 206 and inits associated cell to support the quality of service requirementsassociated with bearer 1.

In response to receiving the acknowledgement message s112, the accessnode A 204 transmits a handover command s114 to the terminal device 608.The handover command s114 contains details of radio resources to be usedin the target cell associated with the access node B 206. The handovercommand s114 may also indicate that the handover is to be carried out inrespect of a radio connection for the transmission of data associatedwith bearer 1.

In response to receiving the handover command s114, the terminal device608 establishes a radio connection with the access node B 206. As partof this establishment the terminal device 608 may send a handovercomplete message s116.

Following the establishment of the radio connection between the terminaldevice 608 and the access node B 206, data associated with bearer 1 maybe transmitted directly to the access node B 206 in step s118. Theaccess node B 206 forwards the received data via the connectionestablished at steps s108 and s110 with the core network 202 as shown atstep s120 or to the access node A 204 (e.g. via the connection 210 shownin FIG. 4). Since the handover procedure was performed in respect ofradio connections for bearer 1 only, the radio connections which wereongoing between the terminal device 608 and the access node A 204 forthe transmission of data via bearers 2 and 3 remain ongoing in steps122.

Subsequently the terminal device 608 transmits a further measurementreport s124, based on which the access node A 204 determines that it isappropriate to perform a mobility procedure in respect of the radioconnections used for the transmission of data via bearers 2 and 3. Theaccess node A 204 therefore initiates a handover preparation phase withthe access node B 206 to request the access node B 206 to reserveresources for the establishment of radio connections between theterminal device 608 and the access node B 206, meeting the quality ofservice requirements associated with bearers 2 and 3. This request and apositive response are shown at step s126.

In response to receiving a positive acknowledgement from the access nodeB 206, the access node A 204 transmits a handover command message s128to the terminal device 608. This indicates radio resources to be usedfor the establishment of radio connections with the access node B 206.In response to receiving the handover command s128, the terminal device608 establishes radio connections with the access node B 206. This maybe by means of the transmission of a handover complete message s130 tothe access node B 206.

Once the radio connections between the terminal device 608 and theaccess node B 206 have been established for the transmission of dataassociated with bearers 2 and 3, the terminal device 608 is thereby inRRC connected mode with the access node B 206 and radio connections areestablished for the transmission of data via bearers 1, 2 and 3 as shownat step s132.As described above, the mobility procedures in steps s20and s30 of FIG. 5 need not be identical, but may differ, for example,based on the quality of service requirements of the respective bearers.

Other handover or mobility procedures may therefore be used in respectof one or more of the bearers, without departing from the scope of theherein described techniques.

For example, a decision to initiate a phase of a mobility procedure maybe based on one or more of downlink reference signals (such as thedownlink reference signals s102 in FIG. 7) as measured by the terminaldevice 608, uplink reference signals which may be transmitted by theterminal device 608 and received and measured by one or more of theaccess node A 204 and the access node B 206, a load of one or more ofthe access nodes 204, 206, and a predicted location of the terminaldevice 206. In some embodiments, reports of measurements of referencesignals are transmitted by the access node or terminal device performingthe measurements, to the entity responsible for initiating the mobilityprocedure.

In FIG. 7, only one target access node (the access node B 206) is shown.However, in some embodiments, more than one candidate target access nodemay be identified, based on, for example, measurements of referencesignals as described above. In some embodiments, the source access nodeA 204 may initiate a handover preparation procedure (such as theprocedure initiated in step s106 of FIG. 7 and described above) inrespect of more than one target access node.

In embodiments in which multiple target access nodes are prepared (thatis, are requested to reserve resources for one or more bearers), afurther step comprises determining to which, if any, access node thebearer handover should occur. This may be based on criteria based onmeasurements of downlink reference signals, measurements of uplinkreference signals, the load of one or more of the access nodes(including the source access node and the target access nodes) and apredicted location of the terminal device 206.

The predicted location of the terminal device 206 may be determinedbased on conventional location technologies (such as cellidentification-based, satellite positioning systems) and sensorsintegrated with or connected to the terminal device 206, for example,such as an IMU forming part of a HMD which is integrated with orconnected to the terminal device 206.

Examples of handover procedures based on uplink reference signals, andillustrating the preparation of multiple handover target cells (andhence access nodes) are described in co-pending application EP17150360.0, the contents of which are hereby incorporated by reference.

In some embodiments of the present technique, one or more of themobility procedure may comprise an early handover procedure. Details ofan early handover process suitable for the present techniques may befound in co-pending application EP 17162884.5 which is herebyincorporated by reference.

FIG. 8 illustrates a process in accordance with the present techniquesin which the first mobility procedure comprises an ‘early handover’,resulting in a temporary dual connectivity configuration, while thesecond mobility procedure comprises a break-before-make procedure. Inthe example shown in FIG. 8, the decision to initiate each mobilityprocedure (and within the early handover procedure, each stage of theprocedure) is made by the terminal device 608.

Steps s100 and s102 correspond to the like-numbered steps describedabove in respect of FIG. 7.

At step s202, the terminal device 608 determines that the access node B206 meets certain pre-determined criteria associated with the firststage of the mobility procedure for bearer 1. This may correspond todetermining that a measurement event has been triggered in respect ofaccess node B 206. For the sake of this example, it will be assumed thefirst predefined criterion satisfied in step s202 is associated with ameasurement event whereby it is determined that current radio channelconditions associated with the first candidate target network accessnode 206 are better than radio channel conditions associated with theserving network access node 204 by at least a first threshold amount.The first threshold amount may also be referred to as a first offset(offset 1)

In response to this determination, the terminal device 608 transmits ameasurement report s204 to the access node A 204.

On receiving the measurement report from the terminal device 608 in steps204, the access node A 204 performs a handover preparation for theradio connection associated with bearer 1 at step s206 by communicatingwith the network access node in respect of which the measurement reportwas triggered (in this case the access node B 206) to provide the accessnode B 206 with configuration information relating to the terminaldevice 608, for example the terminal device context and so forth, and toreceive from the access node B 206 an indication of information to beused by the terminal device 608 to connect to the access node B 206, forexample information relating to radio bearer settings and a dedicatedrandom access channel, RACH, preamble that will be reserved for theterminal device 608.

In step s208 the source network access node 204 transmits an earlyhandover command to the terminal device 608 to provide the terminaldevice 608 with an indication of connection information to be used bythe terminal device 608 for establishing a radio connection for thesupport of the transmission of data via bearer 1 to the access node B206. The specific information provided in the early handover commandwill depend on the specific implementation at hand but may include, forexample, an indication of a dedicated random access channel, RACH,preamble.

After having received the connection information relating to the accessnode B 206 in step s208, the terminal device starts to monitor radiochannel conditions associated with the access node B 206, and todetermine whether or not the conditions satisfy a second triggercriterion. This aspect of the processing may broadly correspond withstep s202, but be associated with a different, higher, threshold. Thatis to say, this aspect of the processing may be configured to determinewhen the radio channel conditions associated with the access node B 206become even better than the radio channel conditions associated with thecurrent source network access node (node A 204) by at least a thresholdamount. While the terminal device is doing this, it is also stillmonitoring for whether any other candidate target network access nodessatisfy the first trigger criterion.

In step s210 it is assumed that a second measurement event is triggeredin respect of the access node B 206. That is to say, it is assumed thatthe terminal device 208 determines the radio channel conditionsassociated with the access node B 206 satisfy a predefined secondtrigger criterion. In this example the predefined second criterioncorresponds in type with the predefined first criterion, but isassociated with a different, higher, threshold. That is to say, for theradio channel conditions associated with the access node B 206 tosatisfy the second trigger criterion in step s210, the terminal device608 determines that a quality characteristic associated withmeasurements of radio channel conditions for the access node B 206 isbetter than a corresponding quality characteristic associated withmeasurements of radio channel conditions for the current serving networkaccess node (access node A 204) by an amount which is higher than theamount required to satisfy the first predefined criterion in step s202.This may occur, for example, because the terminal device has continuedmoving towards the access node B 206 so that the path loss between theaccess node B 206 and the terminal device 608 has continued to decrease,thereby improving radio channel conditions even further until theybecome good enough to meet the second predefined trigger criterion.

Thus in step s210 there is a determination that current radio channelconditions associated with the access node B 206 are better than radiochannel conditions associated with the serving network access node A 204by at least a second threshold amount that is greater than the firstthreshold amount. The second threshold amount may also be referred to asa second offset (offset 2). The difference between offset 1 and offset 2may itself be considered an early handover offset. That is to say,offset 2 represents the level of offset required to trigger a handover,whereas the lower offset, offset 1, represents the level of offsetrequired to provide the terminal device with advance connectioninformation (early handover command) in the expectation a handover maylater occur when the second offset is met.

In response to determining the measurement event (second triggercriterion met) is triggered in step s210 in respect of the access node B206, the terminal device transmits a measurement report to indicate thisto the source network access node A 204, as schematically indicated instep s212. This step may in some cases be performed in accordance withconventional signalling techniques.

On receiving the indication the second trigger criterion is met inrespect of the access node B 206 in step s212, the source network accessnode A 204 may determine that the handover to the access node B 206should proceed, and transmit a simplified handover command to theterminal device 608, as schematically indicated in step s214. Thishandover command may be simplified inasmuch as it does not need toconvey connection information needed by the terminal device to connectto the selected candidate target network access node (the access node B206 in this example) which has already been provided to the terminaldevice in the early handover command of step s208.

As schematically indicted in step s216, in response to receiving thesimplified handover command in step s214, the terminal device 608performs the establishment of the radio connection supporting thetransmission of data via bearer 1, based on the connectioninformation/configuration information received in the early handovercommand of step s208 and sends a handover complete to the access node B206 (i.e. to the selected candidate target network access node, thisnetwork access node being selected on the basis of the second triggercriterion being met while the terminal device had stored connectioninformation for this candidate target network access node).

As schematically indicated step s218, the terminal device can thussynchronise to the access node B 206 (if synchronisation is needed) andoperate in an RRC connected mode with respect to the access node B 206.

The establishment of a connection (either directly, or via the accessnode A 204) between the access node B 206 and the core network 202 isnot shown in FIG. 8, but it will be appreciated that this can be carriedout using conventional techniques, for example, during step s206.

In the example shown in FIG. 8, the terminal device 608 is assumed tosupport a dual connectivity configuration, for example because thedevice 608 comprises dual transmitters/receivers (transceivers). Theestablishment of the radio connection between the terminal device 608and the access node B 206 results in the terminal device 608 operatingin a dual connectivity configuration with respect to bearer 1. As such,bearer 1 may be, at step s218, operating in a ‘split bearer’configuration at least during the mobility procedure. Alternatively,bearer 1 may be supported by a plurality of PDCP entities, each PDCPentity having a peer PDCP entity in a different access node; in otherwords, bearer 1 may be supported by simultaneous connections with twodifferent cells (which may be operating at different frequencies).

In FIG. 8, the establishment of a temporary dual connectivityconfiguration to support bearer 1 is shown by means of various handoverpreparation signalling. In some other embodiments the source access node204 may send a “SeNB addition request” message to the target access node206. The target access node 206 may reply with a “SeNB addition Ack”and, in response, the source access node 206 may send an RRCreconfiguration message to the terminal device 206.

After the establishment of the radio connection between the terminaldevice 608 and the access node B 206 for the transmission of data viabearer 1, the terminal device 608 continues to monitor the referencesignals s102. Measurements of the reference signals are assessed in steps220against criteria in a similar manner as described above in respectof steps s202 and s210. If the criteria are met, the terminal device 608transmits a measurement report s222 to the source access node (accessnode A 204).

In response to receiving the measurement report s222, the source accessnode A 204 performs handover preparation for the radio connectionssupporting the transmission of data for bearers 2 and 3 (step s224), andsubsequently transmits a handover command s226 to the terminal device608. The handover command may include resources reserved by the targetaccess node B 206 for the establishment of radio connections for thetransmission of data associated with bearers 2 and 3.

In response to receiving the handover command s226, the terminal device608 establishes the radio connections with the target access node B 206,for the transmission of data associated with bearers 2 and 3, forexample by means of a handover complete message s228.

In some embodiments, when (or immediately before) dual connectivity isestablished (e.g. at step s218), the target network access node B 206assumes the role of SgNB and the source network access node A assumesthe role of MgNB. In some embodiments, the roles are switched (that is,the target network access node B 206 assumes the role of MgNB and thesource network access node A assumes the role of SgNB) substantiallyconcurrently with the establishment of the radio connections between theterminal device 608 and the target access node B 206 which supportbearers 2 and 3 (e.g. at the same time as, or responsive to, step s228).

Following the establishment of the radio connections for thetransmission of data for bearers 2 and 3, the terminal device operatesbearers 2 and 3 in a single connectivity mode, while bearer 1 remains ina split bearer configuration (step s230).

Subsequently, based on, for example, measurements of uplinktransmissions by the terminal device 608, the terminal device 608 isconfigured to operate bearer 1 in single connectivity mode. As such, theradio connection between the terminal device 608 and the source accessnode A 204 is released. This may be by means of an RRC reconfigurationmessage s232, transmitted from the target access node B 206 to theterminal device 608.

As described above, in some embodiments, a mobility procedure may resultin one or more bearers temporarily being operated in a dual connectivityconfiguration. In dual connectivity, network access nodes are specifiedas being either master network access nodes or secondary network accessnodes, and user equipment can connect to the network through both masterand secondary network access nodes at the same time.

In order to be able to handle a message carried by a split bearer, aterminal device (such as the terminal device 608) is provided with twomedium access control (MAC) entities: a master cell group MAC (MCG MAC)and a secondary cell group MAC (SCG MAC), plus corresponding radio linkcontrol (RLC) and packet data convergence protocol (PDCP). These areincluded in the resources of the terminal device for split bearerhandling.

For terminal devices configured for dual connectivity and split bearertransport, user traffic from the core network can be received at themaster eNB (MeNB) or master gNB (MgNB) as a split bearer, and thendivided between the MeNB and the secondary eNB (SeNB) (or the MgNB andsecondary gNB (SgNB)) for handling and passing to the terminal device.Alternatively, in other split bearer configurations, the data may betransmitted to or from the core network from/to the SgNB or SeNB (inother words, the bearer is ‘split’ at the SgNB/SeNB). Any traffic on aSCG bearer is received from the core network at the SeNB and transportedusing resources of the SeNB to the terminal device.

It has been recognised that the use of dual connectivity to support aterminal device during handover may involve a secondary network accessnode switching roles to become a master network access node, see, forexample, “SgNB to MgNB reconfiguration for 0 ms interruption handover”,3GPP TSG-RAN WG2 #97bis, R2-1703381, Spokane, USA, 3-7 Apr. 2017 [3].

In some embodiments, different keys are assigned to different bearers orgroups of bearers. Specifically, any two bearers which may be thesubject of different mobility procedures do not share the same key.Similarly, a bearer which may be the subject of a dual connectivityconfiguration as part of its respective mobility procedure may beassigned keys which are distinct from keys used for bearers for whichdual connectivity is not used as part of the mobility procedure.

For example, in FIG. 8, the keys used for bearer 1 may be different fromthose used for bearers 2 and 3; the keys used for bearers 2 and 3 may bethe same or different.

In some embodiments of the present technique, each bearer is assignedits own key prior to the first mobility procedure (s20 in FIG. 5).

In some embodiments of the present technique, the key associated with agiven bearer is not changed during the mobility procedures. For example,bearer 1 has KeNB 1 and bearers 2 and 3 have, respectively, KeNB 2 andKeNB 3. In some embodiments, the source access node may indicate whethera key change is needed during HO whether or not different centralisedunits are involved; furthermore, in some embodiments, the source accessnode may indicate that no key change is required in respect of one ormore of the bearers.

Alternatively, in some embodiments, a conventional key hierarchy(whereby each KeNB is specific to a terminal device while in a givencell) is used, but with the addition of a bearer-specific parameter tothe security algorithm. In some such embodiments, the KeNB for aterminal device is not changed during a mobility procedure or dualconnectivity configuration establishment or during role switching by thesource and target access nodes when the terminal device is operatingusing a dual connectivity configuration. The source network access nodemay indicate explicitly whether an existing KeNB remains valid or not;this indication may be at a per-bearer level of granularity.

In some embodiments of the present technique, each bearer has its ownsecurity keys. For example, bearer 1 has its own set of keys which mayor may not be updated when a radio connection with the target accessnode is established.

Other bearers may apply either conventional security keys hierarchy orhave individual keys.

In some embodiments, therefore, it is not necessary to change securitykeys for all the bearers as part of a mobility procedure. In otherwords, for example, the use of a bearer specific key avoids the need tomaintain two keys for the same set of bearers when involved in a earlyhandover or dual connectivity.

Examples of techniques by which the target access node B 206 transitionsfrom SgNB to MgNB may be completed for bearers which operate in dualconnectivity or split bearer configuration during the handover aredescribed in co-pending application EP 17163988.3, which is herebyincorporated by reference in its entirety.

According to such techniques, the transition in respect of a bearer(such as, for example, bearer 1 in FIG. 8) may occur as described below.Although the description is provided in terms of keys derived accordingto LTE principles, it will be readily apparent that the same method oftransition can be applied when one or both of the source and targetaccess nodes operate according to a New Radio (NR) technology.

In a dual connectivity context the master network access node may use akey designated KeNB and the secondary network access node may use a keydesignated SKeNB. The SKeNB is generated by the master network accessnode and terminal device using a key derivation function having the KeNBand a secondary cell group counter as inputs (it does not use a cellID/PCI as an input). The SKeNB is passed from the master network accessnode to the secondary network access node when adding a secondary cellgroup. The SKeNB is then used by the secondary network access node andterminal device to generate KUPenc (key for user plane encryption) forthe secondary network access node. Further details can be found in ETSITS 136 300 V13.2.0 (2016-01)/3GPP TS 36.300 version 13.2.0 Release 13[4], the contents of which are herein incorporated by reference, and inparticular FIG. 14.1-2.

Initially (e g immediately after step s216 of FIG. 8), the sourcenetwork access node 204 is configured as a master network access nodefor the dual connectivity and is associated with a master network accessnode security key (e.g. a KeNB) and the target network access node isconfigured as a secondary network access node for the dual connectivityand is associated with a secondary network access node security key(e.g. a SKeNB). The derivation and sharing of the master network accessnode security key and the secondary network access node security key maybe based on conventional techniques. For example, the source networkaccess node may derive the secondary network access node security keyfrom the master network access node security key in accordance withestablished practices, for example using a key derivation function, andmay convey an indication of the secondary network access node securitykey to the target network access node (for example, at step s206) andthe terminal device (for example, at step s208). It will be appreciatedthat in some implementations the secondary network access node securitykey itself may not be sent from the source network access node 204 tothe target network access node 206 and/or the terminal device 608, butrather an indication that allows the target network access node and theterminal device to independently derive the secondary network accessmode security key may be sent instead. If the secondary network accessnode security key itself is sent, it will typically be encrypted.Nonetheless, for terminological convenience, the process of conveyinginformation to allow a security key to be derived may be referred toherein as simply sending the security key.

Subsequently (e.g. in response to message s222) it is determined thecurrent dual connectivity role of the target network access node (i.e.the network access node currently acting as a secondary network accessnode for the dual connectivity with the terminal device) should beswitched, and in particular the source network access node for thehandover should be configured as a new master network access forsupporting dual connectivity with the terminal device. This may be basedon determining that the terminal device is moving away from the coverageof the first network access node (currently acting as master networkaccess node) and towards the coverage of a further network access node.The decision may be made by either one of the source and target networkaccess nodes according to the implementation at hand.

The target network access node (e.g. the network access node B 206 whichis to switch from acting as a secondary network access node to a masternetwork access node) derives a new master network access node security(KeNB) to be used by the target network access node when it becomesmaster network access node.

The target network access node conveys an indication of the new masternetwork access node security key to the terminal device 608 and the corenetwork 202, thereby allowing the target network access node to switchfrom operating as a secondary network access node to a master networkaccess node for the terminal device, in respect of data transmitted viabearer 1. It will be appreciated that as with SKeNB, the new masternetwork access node security key itself may not be sent from the sourcenetwork access node 204 to the target network access node 206 and/or theterminal device 608, but rather an indication that allows the targetnetwork access node and the terminal device to independently derive thenew master network access mode security key may be sent instead. To theextent any security keys are sent between elements of the wirelesscommunications system, they may be encrypted. Nonetheless, and asalready mentioned above, for terminological convenience, the process ofconveying information to allow a security key to be derived may bereferred to herein as simply sending the security key.

Semi-persistent scheduling (SPS) provides an efficient means to schedulea series of transmissions and is therefore suitable for applicationswith periodic, predictable traffic patterns. An SPS configuration is bydefault associated with the bearer having a highest priority. However,in some scenarios, a terminal device may be configured with more thanone bearer having an equally high priority level. For example, a bearerfor voice-over-IP (VoIP) traffic and a bearer for IMU data may beconfigured to have the same (highest) priority.

According to some embodiments, in order to specify the transmissions forwhich SPS-assigned resources are intended to be used, an access node mayindicate to the terminal device that SPS-assigned resources are to beassociated with a logical channel identity (ID), which is associatedwith a particular RLC or MAC protocol entity.

In response to this indication, the terminal device may select uplinktraffic for transmission on a resource which has been allocated by meansof SPS, the selection being based on the indicated logical channel ID.

In conventional mobility procedures, a single procedure is carried outfor each cell transition by a terminal device. As will be apparent fromthe above, in embodiments of the present technique, two or more mobilityprocedures are applied in respect of a given transition from one cell toanother by a given terminal device. The two or more procedures may eachapply to radio connections associated with (in other words, supportingthe transmission of data for) one or more bearers. Preferably, theselection of mobility procedure, the criteria for triggering eachmobility procedure and assignment to the respective bearer(s) is basedon the quality of service requirements associated with each bearer.

One or more of the mobility procedures may be based on an earlyhandover, wherein, following an initial determination to prepare thehandover, the terminal device receives an indication of resourcesreserved in the target cell and, following a subsequent determination toproceed, receives a command to perform the handover to the target cell.

One or more of the mobility procedures may comprise establishing one ormore bearers in a dual connectivity or split bearer configuration.Preferably, such a mobility procedure is used for radio connectionsassociated with bearers having an extremely low latency tolerance and/orvery low packet loss requirement.

In some embodiments, bearer 1 may be configured in a dual connectivitymode, supported by radio connections between the terminal device and,respectively, a source access node (such as the access node A 206) and afurther access node (not illustrated). The mobility procedures of, forexample, FIG. 5 as described above may, in such embodiments, comprisethe termination of the radio connections (supporting all bearers)between the terminal device and the a source access node and theestablishment of radio connections between the terminal device and the atarget access node to support all bearers. In such embodiments, bearer 1may remain in a dual connectivity configuration throughout theprocedure, and any radio connections between the terminal device and thefurther access node remain established. In some such embodiments, thetarget access node takes on the role of MgNB in respect of the dualconnectivity configuration for bearer 1 substantially at the same timethat the handover of other bearers is completed.

In some embodiments, one or more bearers may be already established in adual connectivity configuration prior to the first of the mobilityprocedures described herein. In addition, or alternatively, one or morebearers may remain in a dual connectivity configuration after thecompletion of the mobility procedures. In some such embodiments, thetransition of an access node from the MgNB role to the SgNB role maycomprise one of the respective mobility procedures described above, forexample in the description of FIG. 5.

In some embodiments of the present technique, different security keysare generated for use by different data. Preferably, bearers which aresubject to different mobility procedures do not use the same keys. Insuch embodiments, where a mobility procedure comprises establishing oneor more bearers in a dual connectivity or split bearer configuration,the target access node transitions from secondary network access node tomaster network access node prior to, or at, the completion of themobility procedure. In some embodiments, the transition from secondarynetwork access node to master network access node in respect of a firstbearer occurs substantially simultaneously with the establishment of aradio connection associated with a second bearer between the terminaldevice and the target access node.

In some embodiments of the present technique, one or more mobilityprocedures, which are performed in respect of a subset of the bearersconfigured for a terminal device, may make use of techniquesconventionally applied to handovers of all bearers for a terminaldevice. For example, a mobility procedure may use a RACH-less handoverand/or a make-before-break handover.

Where one or more mobility procedures comprise multiple stages (e.g.,where each stage is triggered based on assessment of measured radioconditions), a single assessment, using common criteria, may trigger astage (or the entirety) of more than one mobility procedure. Forexample, in the example illustrated in FIG. 8 above, steps s210 and s220may comprise a single assessment. In such an example, steps of theprocedure may be combined and/or re-ordered. For example, where the samecriteria are used in steps s210 and s220 then steps s212 and s222 may becombined into a single measurement report, steps s226 and s214 may becombined into a single message (transmitted after step s224 iscompleted), and steps s216 and s228 may be combined into a singlemessage.

In another example, the determination that a radio connection with atarget access node is to be established for one bearer, and thedetermination to initiate handover preparation (i.e. the request to thetarget access node to reserve resources) for a different bearer may bebased on the same criteria.

In some embodiments, the criteria for triggering a mobility procedure(or phase thereof) are defined in relative terms, relative to criteriafor triggering a different mobility procedure (or different phase of thesame mobility procedure). For example, referring to FIG. 8, the criteriafor the determination at step s202 may be in terms of a received signalreference power of reference signals received from access node A 204falling below a first predetermined threshold; the criteria for thedetermination at step s210 may require that the received signalreference power of reference signals received from access node A 204 hasfallen to 3 dB below the threshold at which the decision at s202 wastriggered.

In some embodiments where a first mobility procedure results in theconfiguration of a bearer in a split-bearer or dual connectivityconfiguration by means of first and second radio connections between theterminal device and the source and target access nodes respectively, thecompletion of a subsequent mobility procedure may trigger the release ofthe radio connection associated with the split bearer between theterminal device and the source access node, thereby removing the splitbearer configuration applicable to the first bearer.

For example, in FIG. 8, in some embodiments the RRC Configurationmessage s232 which reconfigures bearer 1 to no longer operate accordingto a split bearer configuration is not transmitted, and the terminaldevice is configured to release the connection for bearer 1 with thesource access node A 204 at substantially the same time as theestablishment of the radio connections for bearers 2 and 3 with thetarget access node B 206.

The determination to initiate a mobility procedure (or a stage thereof)may be made by the terminal device 608 or the source access node A 204,based on one or more of: measurements of radio transmissions by thesource access node A 204 received at the terminal device 608;measurements of radio transmissions by the target access node A 206received at the terminal device 608; measurements of radio transmissionsby the terminal device 608 received at the source access node A 204;radio resource management (RRM) policies at the source access node A204; traffic load at the source access node A 204. Different criteriamay apply for different mobility procedures (or stages thereof) and therespective determinations at to whether to proceed may be made bydifferent entities.

In FIG. 5, the procedure is illustrated with reference to two bearersand two mobility procedures; in FIGS. 6 and 7, embodiments areillustrated in which two mobility procedures are used for a terminaldevice configured with three bearers. The number of mobility proceduresand bearers is not limited, and embodiments within the scope of thetechniques described herein may use any number (greater than one) ofmobility procedures.

It will be appreciated that the procedures illustrated in FIG. 6, FIG. 7and FIG. 8 comprise many steps which may be omitted, combined orre-ordered, and that in some embodiments, steps or features illustratedin one or more of FIG. 6, FIG. 7 and FIG. 8 may be combined. Forexample, the process in FIG. 8 may be modified so that, as shown in FIG.7, some or all of the determinations which result in the mobilityprocedures being triggered are carried out by the access node A 204,instead of by the terminal device 608.

It will be appreciated that while the present disclosure has in somerespects focused on implementations in an LTE-based and/or 5G networkfor the sake of providing specific examples, the same principles can beapplied to other wireless communications systems. Thus, even though theterminology used herein is generally the same or similar to that of theLTE and 5G standards, the teachings are not limited to the presentversions of LTE and 5G and could apply equally to any appropriatearrangement not based on LTE or 5G and/or compliant with any otherfuture version of an LTE, 5G or other standard.

It may be noted various example approaches discussed herein may rely oninformation which is predetermined/predefined in the sense of beingknown by both the base station and the terminal device. It will beappreciated such predetermined/predefined information may in general beestablished, for example, by definition in an operating standard for thewireless telecommunication system, or in previously exchanged signallingbetween the base station and terminal devices, for example in systeminformation signalling, or in association with radio resource controlsetup signalling. That is to say, the specific manner in which therelevant predefined information is established and shared between thevarious elements of the wireless communications system is notsignificance to the principles of operation described herein. It mayfurther be noted various example approaches discussed herein rely oninformation which is exchanged/communicated between various elements ofthe wireless communications system and it will be appreciated suchcommunications may in general be made in accordance with conventionaltechniques, for example in terms of specific signalling protocols andthe type of communication channel used, unless the context demandsotherwise. That is to say, the specific manner in which the relevantinformation is exchanged between the various elements of the wirelesscommunications system is not significance to the principles of operationdescribed herein.

Respective features of the present disclosure are defined by thefollowing numbered paragraphs:

Paragraph 1. A method of operating a terminal device in a wirelesscommunication network, the wireless communication network comprising afirst network access node associated with a first cell of the wirelesscommunication network and a second network access node associated with asecond cell of the wireless communication network, the methodcomprising:

-   -   establishing, by the terminal device, a first radio connection        between the terminal device and the first network access node        for supporting the transmission of data via a first bearer and a        second radio connection between the terminal device and the        first network access node for supporting the transmission of        data via a second bearer;    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer,        establishing a third radio connection between the terminal        device and the second network access node for supporting the        transmission of the data via the first bearer; and    -   after establishing the third radio connection between the        terminal device and the second network access node, establishing        a fourth radio connection between the terminal device and the        second network access node for supporting the transmission of        the data via the second bearer.

Paragraph 2. The method of paragraph 1, comprising, prior toestablishing the fourth radio connection, configuring the terminaldevice to operate the first bearer in a dual connectivity mode whereinthe first and second radio connections combine to support thetransmission of the data via the first bearer.

Paragraph 3. The method of paragraph 2, comprising

-   -   releasing, by the terminal device, the first radio connection        with the first network access node when the fourth radio        connection with the second network access node is established.

Paragraph 4. The method of paragraph 2, comprising

-   -   configuring the terminal device to continue to operate the first        bearer in the dual connectivity mode after establishing the        fourth radio connection with the second network access node, and        after establishing the fourth radio connection with the second        network access node, releasing, by the terminal device, the        first radio connection with the first network access node.

Paragraph 5. The method of any of paragraphs 1 to 4, wherein

-   -   establishing the third radio connection between the terminal        device and the second network access node is in response to a        first predetermined criteria being satisfied; and    -   establishing the fourth radio connection between the terminal        device and the second network access node is in response to a        second predetermined criteria being satisfied.

Paragraph 6. The method of any of paragraphs 1 to 5, comprising

-   -   measuring radio channel conditions between the terminal device        and the second network access node;    -   determining if the radio channel conditions for the second        network access node satisfy a third predefined criteria, and if        so, transmitting to the first network access node an indication        that the radio channel conditions for the second network access        node satisfy the third predefined criterion;    -   receiving and storing an indication of connection information        for use by the terminal device for establishing the third radio        connection; and    -   receiving an indication that the terminal device should connect        to the second network access node, wherein    -   establishing the third radio connection between the terminal        device and the second network access node comprises    -   transmitting signalling to the second network access node using        the stored connection information to initiate the third radio        connection to the second network access node in response to        receiving the indication that the terminal device should connect        to the second network access node.

Paragraph 7. A method of operating a first network access nodeassociated with a first cell of a wireless communications network, themethod comprising

-   -   establishing a first radio connection between a terminal device        and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer;    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer,        determining that a first criteria has been met in respect of a        radio communications channel between the terminal device and a        second network access node associated with a second cell of the        wireless communications network,    -   in response to determining that the first criteria has been met,        transmitting to the second network access node a request for the        establishment of a third radio connection between the terminal        device and the second network access node for supporting the        transmission of data via the first bearer,    -   determining that a second criteria has been met in respect of        the radio communications channel between the terminal device and        the second network access node,    -   in response to determining that the second criteria has been        met, transmitting to the second network access node a request        for the establishment of a fourth radio connection between the        terminal device and the second network access node for        supporting the transmission of data via the second bearer.

Paragraph 8. The method of paragraph 7, wherein the determining that afirst criteria has been met in respect of a radio communications channelbetween the terminal device and a second network access node comprisesat least one of receiving a measurement report from the terminal device,measuring an uplink reference signal transmitted by the terminal device,and receiving a report based on measurements of uplink reference signalstransmitted by the terminal device from the second network access node.

Paragraph 9. The method of paragraph 7, comprising

-   -   in response to determining that the first criteria have been        met, transmitting an indication to the terminal device that it        is to operate to operate the first bearer in a dual connectivity        mode wherein the first and second radio connections combine to        support the transmission of the data via the first bearer.

Paragraph 10. The method of any of paragraphs 7 to 9, comprising

-   -   determining that an end-to-end latency requirement associated        with the first bearer is not compatible with a forwarding to the        first network access node of data received from the terminal        device by the second network access node via the third radio        channel; and    -   responsive to the determining, transmitting a request to the        second network access node requesting the second network access        node to establish a connection with a core network entity for        the transmission of the data received via the third radio        channel.

Paragraph 11. The method of any of paragraphs 7 to 10, comprising

-   -   determining for each of a plurality of bearers, a        bearer-specific security key.

Paragraph 12. The method of any of paragraphs 7 to 11, comprising

-   -   indicating to the second network access node that a key which is        valid for the terminal device in the first cell is valid for the        terminal device in the second cell,    -   wherein a centralised unit associated with the first cell is        different from a centralised unit associated with the second        cell.

Paragraph 13. The method of any of paragraphs 7 to 11, comprising

-   -   indicating to the second network access node that a key to be        used for the terminal device in the second cell is to be derived        from a bearer-specific parameter.

Paragraph 14. A method of operating a first network access nodeassociated with a first cell of a wireless communications network, themethod comprising

-   -   establishing a first radio connection between a terminal device        and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer,    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer,        determining that a first criteria has been met in respect of a        radio communications channel between the terminal device and a        second network access node associated with a second cell of the        wireless communications network,    -   in response to determining that the first criteria have been        met, transmitting to the terminal device an indication        instructing the terminal device to establish a third radio        connection for supporting transmission of data via the first        bearer with the second network access node,    -   determining that a second criteria has been met in respect of        the radio communications channel between the terminal device and        the second network access node,    -   in response to determining that the second criteria have been        met, transmitting to the terminal device an indication        instructing the terminal device to establish a fourth radio        connection for supporting transmission of data via the second        bearer.

Paragraph 15. The method of paragraph 14, comprising

-   -   determining that an end-to-end latency requirement associated        with the first bearer is not compatible with a forwarding to the        first network access node of data received from the terminal        device by the second network access node via the third radio        channel; and    -   responsive to the determining, transmitting a request to the        second network access node requesting the second network access        node to establish a connection with a core network entity for        the transmission of the data received via the third radio        channel.

Paragraph 16. A method of operating a second network access nodeassociated with a second cell of a wireless communications network, themethod comprising

-   -   receiving a request from a first network access node associated        with a first cell of the wireless communications network to        establish a first radio channel with a terminal device for        supporting the transmission of data via a first bearer,    -   establishing the first radio channel with the terminal device,    -   after establishing the first radio channel with the terminal        device for the first bearer, receiving a request from the first        network access node to establish a second radio channel with the        terminal device for the transmission of data associated with a        second bearer, and    -   establishing the second radio channel with the terminal device.

Paragraph 17. The method of paragraph 16, comprising

-   -   configuring the second network access node to act as a secondary        network access node for a dual connectivity mode of operation        for the first bearer of the terminal device in which the first        network access node acts as a master network access node and the        second network access node acts as a secondary network access        node,    -   the first network access node being associated with a master        network access node security key for the first bearer and the        second network access node being associated with a secondary        network access node security key for the first bearer,    -   the secondary network access node security key being derived        from the master network access node security key and established        by the second network access node from information received from        the first network access node;    -   establishing, while the second network access node is acting as        secondary network access node for the dual connectivity mode of        operation for the first bearer of the terminal device, that the        second network access node should switch to acting as a master        network access node for the dual connectivity mode of operation        for the first bearer of the terminal device;    -   deriving a new master network access node security key to be        used by the second network access node when it is switched to        acting as a master network access node for the dual connectivity        mode of operation for the first bearer of the terminal device;        and    -   configuring the second network access node to act a master        network access node for the dual connectivity mode of operation        for the terminal device using the new master network access node        security key.

Paragraph 18. The method of any of paragraphs 16 to 17, comprising

-   -   receiving data from the terminal device associated with the        first bearer, and    -   forwarding the data to the first network access node.

Paragraph 19. The method of any of paragraphs 16 to 17, comprising

-   -   determining that an end-to-end latency requirement associated        with the first bearer is not compatible with a forwarding of        data received from the terminal device via the first radio        channel to the first network access node; and    -   responsive to the determining, establishing a connection with a        core network entity for the transmission of the data received        via the first radio channel.

Paragraph 20. The method of any of paragraphs 1 to 19 wherein the firstbearer is associated with an end-to-end latency requirement of less thanaround one millisecond.

Paragraph 21. The method of any of paragraphs 1 to 20 wherein data froman inertial measurement unit of a head-mountable display is transmittedusing the first bearer.

Paragraph 22. The method of any of paragraphs 1 to 21 wherein video datais transmitted using the second bearer.

Paragraph 23. The method of paragraph 22 wherein the video datacomprises at least one of data from a camera coupled to a head-mountabledisplay and data for a video display coupled to the head-mountabledisplay.

Paragraph 24. A terminal device for use in a wireless communicationsystem, the wireless communication system comprising a first networkaccess node associated with a first cell of the wireless communicationnetwork and a second network access node associated with a second cellof the wireless communication system, the terminal device comprisingcontroller circuitry, transmitter circuitry and receiver circuitryconfigured to operate together such that the terminal device isoperable:

-   -   to establish a first radio connection between the terminal        device and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer;    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer, to        establish a third radio connection between the terminal device        and the second network access node for supporting the        transmission of the data via the first bearer; and    -   after establishing the third radio connection between the        terminal device and the second network access node, to establish        a fourth radio connection between the terminal device and the        second network access node for supporting the transmission of        the data via the second bearer.

Paragraph 25. Circuitry for a terminal device for use in a wirelesscommunication system, the wireless communication system comprising afirst network access node associated with a first cell of the wirelesscommunication system and a second network access node associated with asecond cell of the wireless communication system, the circuitrycomprising controller circuitry, transmitter circuitry and receivercircuitry configured to operate together such that the circuitry isoperable:

-   -   to establish a first radio connection between the terminal        device and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer;    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer, to        establish a third radio connection between the terminal device        and the second network access node for supporting the        transmission of the data via the first bearer; and    -   after establishing the third radio connection between the        terminal device and the second network access node, to establish        a fourth radio connection between the terminal device and the        second network access node for supporting the transmission of        the data via the second bearer.

Paragraph 26. A first network access node for use in a wirelesstelecommunication system comprising a terminal device, the first networkaccess node and a second network access node, the first network accessnode comprising controller circuitry and transceiver circuitryconfigured to operate together such that the first network access nodeis operable:

-   -   to establish a first radio connection between the terminal        device and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer;    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer, to        determine that a first criteria has been met in respect of a        radio communications channel between the terminal device and a        second network access node associated with a second cell of the        wireless communications network,    -   in response to determining that the first criteria has been met,        to transmit to the second network access node a request for the        establishment of a third radio connection between the terminal        device and the second network access node for supporting the        transmission of data via the first bearer,    -   to determine that a second criteria has been met in respect of        the radio communications channel between the terminal device and        the second network access node,    -   in response to determining that the second criteria has been        met, to transmit to the second network access node a request for        the establishment of a fourth radio connection between the        terminal device and the second network access node for        supporting the transmission of data via the second bearer.

Paragraph 27. Circuitry for a first network access node for use in awireless telecommunication system comprising a terminal device, thefirst network access node and a second network access node, thecircuitry comprising controller circuitry and transceiver circuitryconfigured to operate together such that the circuitry is operable:

-   -   to establish a first radio connection between the terminal        device and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer;    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer, to        determine that a first criteria has been met in respect of a        radio communications channel between the terminal device and a        second network access node associated with a second cell of the        wireless communications network,    -   in response to determining that the first criteria has been met,        to transmit to the second network access node a request for the        establishment of a third radio connection between the terminal        device and the second network access node for supporting the        transmission of data via the first bearer,    -   to determine that a second criteria has been met in respect of        the radio communications channel between the terminal device and        the second network access node,    -   in response to determining that the second criteria has been        met, to transmit to the second network access node a request for        the establishment of a fourth radio connection between the        terminal device and the second network access node for        supporting the transmission of data via the second bearer.

Paragraph 28. A first network access node for use in a wirelesstelecommunication system comprising a terminal device, the first networkaccess node and a second network access node, the first network accessnode comprising controller circuitry and transceiver circuitryconfigured to operate together such that the first network access nodeis operable:

-   -   to establish a first radio connection between a terminal device        and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer,    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer, to        determine that a first criteria has been met in respect of a        radio communications channel between the terminal device and a        second network access node associated with a second cell of the        wireless communications network,    -   in response to determining that the first criteria have been        met, to transmit to the terminal device an indication        instructing the terminal device to establish a third radio        connection for supporting transmission of data via the first        bearer with the second network access node,    -   to determine that a second criteria has been met in respect of        the radio communications channel between the terminal device and        the second network access node,    -   in response to determining that the second criteria have been        met, to transmit to the terminal device an indication        instructing the terminal device to establish a fourth radio        connection for supporting transmission of data via the second        bearer.

Paragraph 29. Circuitry for a first network access node for use in awireless telecommunication system comprising a terminal device, thefirst network access node and a second network access node, thecircuitry comprising controller circuitry and transceiver circuitryconfigured to operate together such that the circuitry is operable:

-   -   to establish a first radio connection between a terminal device        and the first network access node for supporting the        transmission of data via a first bearer and a second radio        connection between the terminal device and the first network        access node for supporting the transmission of data via a second        bearer,    -   in accordance with a difference between a quality of service for        the data transmission via the first bearer and a quality of        service for the data transmission via the second bearer, to        determine that a first criteria has been met in respect of a        radio communications channel between the terminal device and a        second network access node associated with a second cell of the        wireless communications network,    -   in response to determining that the first criteria have been        met, to transmit to the terminal device an indication        instructing the terminal device to establish a third radio        connection for supporting transmission of data via the first        bearer with the second network access node,    -   to determine that a second criteria has been met in respect of        the radio communications channel between the terminal device and        the second network access node,    -   in response to determining that the second criteria have been        met, to transmit to the terminal device an indication        instructing the terminal device to establish a fourth radio        connection for supporting transmission of data via the second        bearer.

Paragraph 30. A second network access node for use in a wirelesstelecommunication system comprising a terminal device, a first networkaccess node and the second network access node, the second networkaccess node comprising controller circuitry and transceiver circuitryconfigured to operate together such that the first network access nodeis operable:

-   -   to receive a request from a first network access node associated        with a first cell of the wireless communications network to        establish a first radio channel with a terminal device for        supporting the transmission of data via a first bearer,    -   to establish the first radio channel with the terminal device,    -   after establishing the first radio channel with the terminal        device for the first bearer, to receive a request from the first        network access node to establish a second radio channel with the        terminal device for the transmission of data associated with a        second bearer, and    -   to establish the second radio channel with the terminal device.

Paragraph 31. Circuitry for a second network access node for use in awireless telecommunication system comprising a terminal device, a firstnetwork access node and the second network access node, the circuitrycomprising controller circuitry and transceiver circuitry configured tooperate together such that the circuitry is operable:

-   -   to receive a request from a first network access node associated        with a first cell of the wireless communications network to        establish a first radio channel with a terminal device for        supporting the transmission of data via a first bearer,    -   to establish the first radio channel with the terminal device,    -   after establishing the first radio channel with the terminal        device for the first bearer, to receive a request from the first        network access node to establish a second radio channel with the        terminal device for the transmission of data associated with a        second bearer, and    -   to establish the second radio channel with the terminal device.

Paragraph 32. A head-mountable display comprising

-   -   an inertial measurement unit,    -   a terminal device according to paragraph 24,    -   wherein data from the inertial measurement unit is transmitted        using the first bearer.

Paragraph 33. The head-mountable display of paragraph 32 comprising

-   -   at least one of a camera and a video display,    -   wherein video data is transmitted using the second bearer.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

REFERENCES

[1] 3GPP document RP-160671, “New SID Proposal: Study on New RadioAccess Technology,” NTT DOCOMO, RAN#71, Gothenburg, Sweden, 7 to 10 Mar.2016.

[2] Holma H. and Toskala A, “LTE for UMTS OFDMA and SC-FDMA based radioaccess”, John Wiley and Sons, 2009.

[3] “SgNB to MgNB reconfiguration for 0 ms interruption handover”, 3GPPTSG-RAN WG2 #97bis, R2-1703381, submitted for meeting at Spokane, USA,3-7 Apr. 2017

[4] ETSI TS 136 300 V13.2.0 (2016-01)/3GPP TS 36.300 version 13.2.0Release 13

1. A method of operating a terminal device in a wireless communicationnetwork, the wireless communication network comprising a first networkaccess node associated with a first cell of the wireless communicationnetwork and a second network access node associated with a second cellof the wireless communication network, the method comprising:establishing, by the terminal device, a first radio connection betweenthe terminal device and the first network access node for supporting thetransmission of data via a first bearer and a second radio connectionbetween the terminal device and the first network access node forsupporting the transmission of data via a second bearer; in accordancewith a difference between a quality of service for the data transmissionvia the first bearer and a quality of service for the data transmissionvia the second bearer, establishing a third radio connection between theterminal device and the second network access node for supporting thetransmission of the data via the first bearer; and after establishingthe third radio connection between the terminal device and the secondnetwork access node, establishing a fourth radio connection between theterminal device and the second network access node for supporting thetransmission of the data via the second bearer.
 2. The method of claim1, comprising, prior to establishing the fourth radio connection,configuring the terminal device to operate the first bearer in a dualconnectivity mode wherein the first and second radio connections combineto support the transmission of the data via the first bearer.
 3. Themethod of claim 2, comprising releasing, by the terminal device, thefirst radio connection with the first network access node when thefourth radio connection with the second network access node isestablished.
 4. The method of claim 2, comprising configuring theterminal device to continue to operate the first bearer in the dualconnectivity mode after establishing the fourth radio connection withthe second network access node, and after establishing the fourth radioconnection with the second network access node, releasing, by theterminal device, the first radio connection with the first networkaccess node.
 5. The method of claim 1, wherein establishing the thirdradio connection between the terminal device and the second networkaccess node is in response to a first predetermined criteria beingsatisfied; and establishing the fourth radio connection between theterminal device and the second network access node is in response to asecond predetermined criteria being satisfied.
 6. The method of claim 1,comprising measuring radio channel conditions between the terminaldevice and the second network access node; determining if the radiochannel conditions for the second network access node satisfy a thirdpredefined criteria, and if so, transmitting to the first network accessnode an indication that the radio channel conditions for the secondnetwork access node satisfy the third predefined criterion; receivingand storing an indication of connection information for use by theterminal device for establishing the third radio connection; andreceiving an indication that the terminal device should connect to thesecond network access node, wherein establishing the third radioconnection between the terminal device and the second network accessnode comprises transmitting signalling to the second network access nodeusing the stored connection information to initiate the third radioconnection to the second network access node in response to receivingthe indication that the terminal device should connect to the secondnetwork access node.
 7. A method of operating a first network accessnode associated with a first cell of a wireless communications network,the method comprising establishing a first radio connection between aterminal device and the first network access node for supporting thetransmission of data via a first bearer and a second radio connectionbetween the terminal device and the first network access node forsupporting the transmission of data via a second bearer; in accordancewith a difference between a quality of service for the data transmissionvia the first bearer and a quality of service for the data transmissionvia the second bearer, determining that a first criteria has been met inrespect of a radio communications channel between the terminal deviceand a second network access node associated with a second cell of thewireless communications network, in response to determining that thefirst criteria has been met, transmitting to the second network accessnode a request for the establishment of a third radio connection betweenthe terminal device and the second network access node for supportingthe transmission of data via the first bearer, determining that a secondcriteria has been met in respect of the radio communications channelbetween the terminal device and the second network access node, inresponse to determining that the second criteria has been met,transmitting to the second network access node a request for theestablishment of a fourth radio connection between the terminal deviceand the second network access node for supporting the transmission ofdata via the second bearer.
 8. The method of claim 7, wherein thedetermining that a first criteria has been met in respect of a radiocommunications channel between the terminal device and a second networkaccess node comprises at least one of receiving a measurement reportfrom the terminal device, measuring an uplink reference signaltransmitted by the terminal device, and receiving a report based onmeasurements of uplink reference signals transmitted by the terminaldevice from the second network access node.
 9. The method of claim 7,comprising in response to determining that the first criteria have beenmet, transmitting an indication to the terminal device that it is tooperate to operate the first bearer in a dual connectivity mode whereinthe first and second radio connections combine to support thetransmission of the data via the first bearer.
 10. The method of claim7, comprising determining that an end-to-end latency requirementassociated with the first bearer is not compatible with a forwarding tothe first network access node of data received from the terminal deviceby the second network access node via the third radio channel; andresponsive to the determining, transmitting a request to the secondnetwork access node requesting the second network access node toestablish a connection with a core network entity for the transmissionof the data received via the third radio channel.
 11. The method ofclaim 7, comprising determining for each of a plurality of bearers, abearer-specific security key.
 12. The method of claim 7, comprisingindicating to the second network access node that a key which is validfor the terminal device in the first cell is valid for the terminaldevice in the second cell, wherein a centralised unit associated withthe first cell is different from a centralised unit associated with thesecond cell.
 13. The method of claim 7, comprising indicating to thesecond network access node that a key to be used for the terminal devicein the second cell is to be derived from a bearer-specific parameter.14. A method of operating a first network access node associated with afirst cell of a wireless communications network, the method comprisingestablishing a first radio connection between a terminal device and thefirst network access node for supporting the transmission of data via afirst bearer and a second radio connection between the terminal deviceand the first network access node for supporting the transmission ofdata via a second bearer, in accordance with a difference between aquality of service for the data transmission via the first bearer and aquality of service for the data transmission via the second bearer,determining that a first criteria has been met in respect of a radiocommunications channel between the terminal device and a second networkaccess node associated with a second cell of the wireless communicationsnetwork, in response to determining that the first criteria have beenmet, transmitting to the terminal device an indication instructing theterminal device to establish a third radio connection for supportingtransmission of data via the first bearer with the second network accessnode, determining that a second criteria has been met in respect of theradio communications channel between the terminal device and the secondnetwork access node, in response to determining that the second criteriahave been met, transmitting to the terminal device an indicationinstructing the terminal device to establish a fourth radio connectionfor supporting transmission of data via the second bearer.
 15. Themethod of claim 14, comprising determining that an end-to-end latencyrequirement associated with the first bearer is not compatible with aforwarding to the first network access node of data received from theterminal device by the second network access node via the third radiochannel; and responsive to the determining, transmitting a request tothe second network access node requesting the second network access nodeto establish a connection with a core network entity for thetransmission of the data received via the third radio channel. 16.-19.(canceled)
 20. The method of claim 1, wherein the first bearer isassociated with an end-to-end latency requirement of less than aroundone millisecond.
 21. The method of claim 1, wherein data from aninertial measurement unit of a head-mountable display is transmittedusing the first bearer.
 22. The method of claim 1, wherein video data istransmitted using the second bearer.
 23. The method of claim 22 whereinthe video data comprises at least one of data from a camera coupled to ahead-mountable display and data for a video display coupled to thehead-mountable display. 24.-35. (canceled)